PROJECT SUMMARY Epidemic Kaposi?s sarcoma (KS) is an HIV-1 associated tumor, and remains one of the highest incidence neoplasms in sub-Saharan African men and women despite effective antiretroviral therapy (ART) programs. The fundamental mechanism driving KS appears to be KSHV infection, but it is clear that KSHV infection alone is insufficient for KS development since most infected individuals do not develop KS. HIV-1 co-infection or therapeutic immune suppression induces, exacerbates, or accelerates KS disease. How HIV-1 co-infection synergizes with KSHV to form or maintain the tumor niche is largely unexplored in tissue. In many cancers, a clear understanding of the tumor microenvironment including the presence, antigen- specificity and functionality of tumor-infiltrating lymphocytes (TILs) is revolutionizing therapeutic approaches. Yet for KS we have a dearth of information about the nature of TILs and whether the tumor microenvironment is suppressing their anti-neoplastic function in an HIV-1 dependent manner. This project seeks to rectify this knowledge gap in a Case (HIV-1+/KS+) versus Control ((HIV-1+/KS-) design by comparatively investigating the phenotypes and the functionality of TILs in comparison to the same cells in the peripheral immune system and by comparing the tumor and peripheral immune cell expression patterns. Our hypothesis is that even in the face of a detectable KSHV-reactive peripheral T cell response and effective HIV-1 suppression, there are insufficient numbers of KSHV Ag-specifc tumor-infiltrating T lymphocytes (TILs) and these cells are non-responsive to antigen in KS tumors. Our approach is 1) to functionally compare the peripheral CD4 and CD8 T cell responses to KSHV and ubiquitous immunodominant antigens in KS with differential HIV-1 disease duration; 2) to isolate, and immunophenotypically and functionally characterize KS tumor infiltrating leukocytes; and 3) to compare the transcriptomes of TILs with autologous peripheral cells or those from KS asymptomatic controls. From these comparative investigations, we anticipate deriving a more robust understanding of the expression programs and immune responsiveness of adaptive immune cells in the KS tumor niche and more complete understanding of the role of HIV-1, immune suppression, anergy, exhaustion, and senescence in defining that niche. This understanding will direct interventional strategies including the design of immunotherapeutics and potentially vaccines against KS.